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The 49ers can breathe a big sigh of relief: The NFL’s biggest running back received positive medical news after undergoing an MRI exam on Sunday.

According to a league source, the results of Brandon Jacobs‘ exam revealed no major damage to his left knee after he sustained what appeared to be a potentially severe injury in a preseason loss to the Texans on Saturday night.

After the game, before Jacobs underwent his exam, coach Jim Harbaugh acknowledged Jacobs would “miss a little time.” There remains no timetable for Jacobs’ return, meaning his availability for the regular-season opener at Green Bay on Sept. 9 could be in doubt.

On Sunday afternoon, Jacobs had alluded to positive medical results on Twitter.

At 3:47 p.m., Jacobs wrote: “Yeeeeaaaa.”

That was followed less than an hour later by “Good Good Good.”

Signed to a one-year deal in the offseason, the 264-pound Jacobs is expected to serve as a short-yardage back. In the preseason opener, the 49ers handed the ball to Jacobs on four occasions when they needed 1 yard for a first down and he had runs of 3, 23, 3 and 2 yards.

James’ ankle: Rookie running back LaMichael James said he “tweaked” his left ankle in the preseason opener before sustaining a more severe injury to the same ankle Saturday.

James didn’t miss a practice last week due to the injury. And he insisted after Saturday’s game that he wouldn’t miss Monday’s practice, despite being taken to the X-ray room on a cart.

James said his body went in one direction and his ankle remained planted when 49ers tackle Derek Hall fell on him while James was blocking in the fourth quarter.

“You want to finish a game,” James said. “You never want to lay out on the field. I think that shows weakness. I don’t want to be that guy.”

Regular-season mode: Just a preseason game? You couldn’t tell by watching Harbaugh, who goes at full throttle whenever a scoreboard is involved. Harbaugh got worked up yelling at the replacement officials, and Texans left tackle Duane Brown indicated the 49ers took on the personality of their coach.

“They came in here not with the mind-set of a preseason game, but a regular-season game,” Brown said. “I think that really helped us out a lot. It helped us gauge where we are, what we need to work on and bulks us up a little bit. Preseason, we can kind of get into slo-mo depending on how teams come in playing. They came in here with a different mentality.”

Comfort zone: For the second straight week, backup quarterback Colin Kaepernick‘s second play of the game was a read-option fake to James followed by a run to the outside.

And for the second straight week, it worked.

In the preseason opener, Kaepernick sprinted 78 yards for a touchdown down the right sideline. On Saturday, he ran 12 yards down the left side. On the next play, he handed off to James, who picked up 8 yards.

Based on its effectiveness, it wouldn’t be surprising if the 49ers employed an option package with Kaepernick in the regular season.

“That’s something they asked me to do the last couple of weeks,” said Kaepernick, the only quarterback in NCAA history with three straight 1,000-yard rushing seasons. “It’s similar to what I did in college, so obviously I’m comfortable with that.”